1. Field of the Invention
The invention pertains to a spring strut unit for suspension systems of motor vehicles with a fastening connection on the body side and another fastening connection on the wheel side; with a helical spring supported between the body and the wheel; and with a vibration damper installed between the body-side fastening connection and the wheel-side fastening connection, where the vibration damper consists of a working cylinder, a damping piston attached to a piston rod, damping valves installed in the damping piston, and a compensating space, and where the helical spring can be pretensioned by a ring-shaped cylinder, which can be actuated by a hydraulic medium.
2. Description of the Related Art
Spring strut units for suspension systems of motor vehicles are already known in which a vibration damper is coaxially surrounded by a helical spring and where the helical spring is supported at one end against a fastening element of the piston rod and at the other end against a spring plate in the area of the outer cylinder. The spring plate is coaxial to the vibration damper and can be filled with hydraulic medium from an external source by way of a hydraulic piston-cylinder assembly, so that the helical spring can be subjected to the desired pretension. The spring plate is thus hydraulically adjustable, the pressure being generated externally.
Another self-pumping, height-adjustable spring strut unit for vehicles in which the spring plate of a helical spring is designed to be hydraulically adjustable is known from GB 2,164,417. The hydraulic adjustment is accomplished by the relative movement between the vehicle body and the vehicle axle, but this device requires electromagnetic control valves and an electrically operated distance sensor to regulate the level of the body. The previously mentioned designs are therefore dependent on outside energy to realize a fully automatic level control function and therefore have a correspondingly complex design. In particular, electronic sensing equipment and automatic controllers are liable to malfunction during the operation of the vehicle.